The present invention relates to an orientative ferroelectric thin film which can be used in the case of making up a non-volatile memory, a capacitor, an optical modulation element or the like on a semiconductor substrate.
Conventionally, owing to various properties of a ferroelectric substance such as a ferro-electric property, a piezo-electric property, a pyro-electric property and an electro-optic effect, an oxide ferroelectric thin film has been expected to be applied not only to a non-volatile memory but also to various devices such as a surface elastic wave element, an infrared ray pyro-electric element, an acoustic optical element, an electro-optic element. Of these applications, in order to realize low light loss in a thin film optical waveguide structure and to obtain polarization property or electro-optic effect comparable to a single crystal, it is inevitable to make up a single crystal thin film. Therefore, a number of attempts have been made to make up an epitaxial ferroelectric thin film of BaTiO.sub.3, PbTiO.sub.3, Pb.sub.1-x La.sub.x (Zr.sub.1-y Ti.sub.y).sub.1-x/4 O.sub.3 (PLZT) where 0.ltoreq.x, y.ltoreq.1, LiNbO.sub.3, KNbO.sub.3, Bi.sub.4 Ti.sub.3 O.sub.12 or the like on an oxide single crystal substrate by such a method as Rf-magnetron sputtering, ion beam sputtering, laser ablation or organo-metal chemical vapor deposition (MOCVD).
However, in order to achieve integration with semiconductor elements, it is necessary to make up a ferroelectric thin film on a semiconductor substrate. However, it is difficult to attain the epitaxial growth of a ferroelectric thin film on a semiconductor substrate such as a Si substrate, because of high temperature in the growth, mutual diffusion between a semiconductor and a ferroelectric substance, the oxidation of the semiconductor and so on. Also, the epitaxial growth of a ferroelectric thin film onto a GaAs substrate is difficult for the following reasons. That is, GaAs is known as a substance in which As of the surface is reduced at 400.degree. C. or more, and with no As.sub.4 atmosphere, As and Ga begin to be sublimated for each layer at 690.degree. C. or more. Although reports about making up a ferroelectric thin film onto a GaAs substrate, are extremely few in number, it is known that the diffusion of Pb to GaAs is detected when PLZT has grown up on a GaAs substrate.
For these reasons, it is necessary to form a capping layer as a buffer layer on a semiconductor substrate so that the capping layer helps the epitaxial growth of a ferroelectric thin film and acts as a diffusion barrier. Further, if such a buffer layer is provided in an FET element in which an insulator is formed between a ferroelectric substance and a semiconductor, it is possible to prevent charges from being injected from the semiconductor at the time of the polarization of the ferroelectric substance, so that it is easy to maintain the polarization state of the ferroelectric substance. A ferroelectric substance generally has a smaller refractive index than Si and GaAs. If a buffer layer having a refractive index smaller than a ferroelectric substance can be obtained, it is possible to confine semiconductor laser light in a ferroelectric thin film optical waveguide, so that it is possible to make up an optical modulation element on a semiconductor laser or to produce an optical integrated circuit on an Si semiconductor integrated circuit.
On the other hand, Japanese Patent Unexamined Publication No. Sho-61-185808 discloses a technique in which a ferroelectric compound is grown up epitaxially on a substrate in which MgAl.sub.2 O.sub.4 (100) or MgO (100) has been grown up epitaxially as a buffer layer on an Si (100) single crystal. However, as shown in the following Table 1, the lattice constants of MgAl.sub.2 O.sub.4 and MgO are 8.083 angstrom (1/2 is 4.042), and 4.213 angstrom respectively, so that it has been difficult to epitaxially grow up a typical ferroelectric substances such as PbTiO.sub.3 (lattice constant a=3.899 angstrom, c=4.153 angstrom), BaTiO.sub.3 (lattice constant a=3.994 angstrom, c=4.038 angstrom) in c-axis orientation on the above-mentioned MgAl.sub.2 O.sub.4 or MgO since the lattice constant of each of MgAl.sub.2 O.sub.4 and MgO is closer to the lattice constant of the c-axis of the ferroelectric substance than to that of the a-axis, or shows a value between the lattice constants of the both axes. Therefore, if PiTiO.sub.3, BaTiO.sub.3 or the like is grown up on epitaxial MgAl.sub.2 O.sub.4 (100) or MgO (100) on an Si (100) single crystal substrate, it has been possible to obtain only an orientation film in which the (100) plane and (001) plane of such a ferroelectric substrance are mixed with each other and orientated in parallel to the substrate, that is, a-axis orientation and c-axis orientation crystal grains are mixed.
Table 1 shows the lattice constants of SrTiO.sub.3, MgAl.sub.2 O.sub.4 and MgO as buffer layer and PbTiO.sub.3 and BaTiO.sub.3 as ferro-electric substance, and the relationship of lattice mismatching degree between them.
TABLE 1 __________________________________________________________________________ lattice BaTiO.sub.3 PbTiO.sub.3 constant a (.ANG.) c (.ANG.) a (.ANG.) c (.ANG.) crystal structure structure (.ANG.) 3.994 4.038 3.889 4.153 __________________________________________________________________________ SrTiO.sub.3 perov- cubic 3.905 2.3% 3.4% -0.2% 6.4% skite MgAl.sub.2 O.sub.4 spinal cubic 8.083 -1.2% -0.1% -3.5% 2.8% (1/2 = 4.042) MgO NaCl cubic 4.213 -5.2% -4.2% -7.5% -1.4% __________________________________________________________________________
Further, in the above mentioned Japanese Patent Unexamined Publication No. Sho-61-185808, the crystallographic relationship between the Si (100) single crystal and the MgO (100) was not shown obviously therein. In fact, according to the study after that, it was made clear that MgO was an orientative poly-crystal MgO having a random intra-surface orientation although its (100) plane was parallel to the Si (100) plane (P. Tiwari et al., J. Appl. Phys. 69, 8358 (1991)).
After that, it was turned out for the first time that MgO which was often used as a substrate of a ferroelectric substance or a high temperature superconductor because of its lattice constant and thermal stability, could be grown up epitaxially on Si (D. K. Fork et al., Appl. Phys. Lett. 58, 2294 (1991)), and there has been proposed a superconductor thin film using this fact.
However, as described above, it has been difficult in the prior art to form an epitaxial or orientative ferroelectric thin film of c-axis orientation on a semiconductor substrate. Further, it has been difficult in the prior art to grow up a ferroelectric thin film on a single crystal Si substrate epitaxially.